The present invention relates to dynamoelectric machines and, more particularly, to rotors for such machines and to a method of making the same.
It has long been observed that the vibration of AC motors/generators appears to increase in amplitude as they heat up with applied load. One cause of such vibration appears to be a balance shift in the rotor as it heats up due to resistance losses in the rotor bars and eddy current losses in the magnetic steel laminations.
Rotors of large AC induction motors are conventionally formed by stacking thin laminations of magnetic steel on a mandrel, inserting rotor conductor bars in axial slots passing through the laminations, axially compressing the stack and welding the ends of the conductive rotor bars to end rings which then maintain the stack in its compressed condition. The rotor stack is fixedly mounted on a shaft both by a thermal shrink-fit and by a keyway in the stack and the shaft.
Rotors of large AC induction motors can be roughly categorized as slow-speed rotors which are suitable for slow-speed machines and high-speed rotors which are suitable for high-speed machines. Typically the outside diameter of a slow-speed rotor is relatively large. Such large diameter permits the inclusion of a plurality of axial cooling holes in the rotor and radial cooling slots communicating the axial cooling holes to the outer diameter of the rotor. The rotors of high-speed machines such as, for example, of two-pole induction motors which rotate at about 3600 RPM, are necessarily of substantially smaller diameter than those of slow-speed machines. In a smaller-diameter rotor, axial holes through the stacked magnetic steel laminations may adversely affect the magnetic properties of the rotor. Accordingly, it is conventional to build high-speed rotors as substantially solid stacks of laminations without either axial or radial cooling holes.
We have observed that rotor imbalance in high-speed induction motors appears to be more related to rotor temperature than is the case with the larger slow-speed rotors with cooling holes and slots. We have discovered that the shrink-fit contact pressure between the stacked laminations and the shaft is less evenly distributed and less uniform in the high-speed rotor. In addition, one type of imbalance we have observed in assembled high-speed rotors appears to be in consistent angular relationship with the keyway.
Although we do not intend to be bound by a particular theory of why the possible causes of thermally related imbalance appear to be aggravated in high-speed rotors, we propose the following theory.
During shrink fitting of the substantially solid (that is, one not containing axial cooling holes) stack to a shaft, the higher radial rigidity of the solid laminations produces a higher interface pressure on the shaft at some relatively uncontrolled locations with other locations and a relatively light interface pressure at other locations. When the rotor bars of a solid high-speed rotor expand and possibly shift locations slightly with increased temperature during operation, the contact pressure tends to become redistributed between high and low contact pressure regions. This produces a change in rotor balance. Such change in rotor balance occurring in operation this way takes place after the normal rotor balancing operations are completed and thus may lead to rotor vibration in the field.
Another cause of imbalance appears to arise from tiny burrs in the punchings in the vicinity of the keyway. Before assembly, the shaft is accurately straightened. The die used to punch the keyway in each punching tends to leave a consistent burr pattern in each lamination. When the laminations are stacked to form the stack, any consistent burrs formed at the keyway during fabrication are aligned along the axis to thus produce a greater stack length at the keyway than at other locations around the circumference of the stack. For example, if a burr only 0.0001 inch thick is present at the keyway on, for example, 1200 laminations, the assembled stack is longer by 0.12 inch in the vicinity of the keyway. When the end rings are welded to the rotor bars, this greater stack thickness along the keyway produces greater axial tension in the vicinity of the keyway and thus tends to bend the shaft. This bend is then locked in by the shrink fit. The shaft may again be straightened after assembly, but the bend remains in the shrink-fit contact area. Changes in tension by thermal expansion of the rotor bars combined with the non-uniform shrink-fit contact pressure can permit some of the laminations to migrate and to thereby deform the shaft and/or stack and cause its shape and balance to change relative to its condition upon completion of manufacturing. If the condition of shape and balance were to remain stable instead of changing, balancing at completion of manufacturing is capable of remedying the imbalance. It is the change of shape and/or balance that constitutes the major problem.